Negative thermal expansion (contraction with increasing temperature) is an unusual and potentially useful property for a solid material, and very few crystalline materials possess strongly negative expansions over an extended temperature range. Materials that exhibit a negative expansion due to extensive microcracking, by virtue of a negative coefficient of thermal expansion ("CTE") along at least one crystallographic axis and a substantially different CTE along at least one other axis, include some lithium aluminosilicates, "NZPs" (compounds with crystal structures similar to that of NaZr.sub.2 P.sub.3 O.sub.12), and Ta.sub.2 O.sub.5 --WO.sub.3 compounds. On the other hand, materials having negative mean lattice expansions that do not require microcracking for negative bulk ceramic CTEs are even more limited, and include certain synthetic alkali-free zeolites having low aluminum contents, ZrW.sub.2 O.sub.8, HfW.sub.2 O.sub.8, ZrV.sub.2-x P.sub.x O.sub.7 (above about 100.degree. C.), and NbZrP.sub.3 O.sub.12 (an NZP type compound).
Of the compounds that have negative mean lattice expansions, zeolite expansions from 25 to 100.degree. C. are in the range -20.times.10.sup.-7.degree. C..sup.-1 to -40.times.10.sup.-7.degree. C..sup.-1, but have the disadvantages of being dependent upon the amount of adsorbed water in the zeolite, and their CTE curves can exhibit considerable hysteresis. The CTE of NbZrP.sub.3 O.sub.12 below 100.degree. C. is about -27.times.10.sup.-7.degree. C..sup.-1, while that of ZrW.sub.2 O.sub.8 and HfW.sub.2 O.sub.8 is about -90.times.10.sup.-7.degree. C..sup.-1. Thus, ZrW.sub.2 O.sub.8 and HfW.sub.2 O.sub.8 are presently unique as materials that display a CTE more negative than -40.times.10.sup.-7.degree. C..sup.-1 without microcracking.
Martinek and Hummel (1960, J. Amer. Ceram. Soc., 53, 159-161) first reported the existence of Zr.sub.2 P.sub.2 WO.sub.12 in their study of the phase relations in the ZrO.sub.2 --WO.sub.3 --P.sub.2 O.sub.5 system at 1125.degree. C. An XRD powder pattern was presented for this new compound, which reportedly has a melting point above 1750.degree. C., although extensive volatilization occurs in air at 1600.degree. C. Synthesis of Zr.sub.2 P.sub.2 WO.sub.12 was achieved by calcining a mixture of hydrous zirconium carbonate, tungstic acid, and mono-hydrogen ammonium phosphate.
Tsvigunov and Sirotinkin (1990, Russ. Jour. of Inorg. Chem., 35, 1740) subsequently reported a more complete and precise powder XRD pattern for this compound, which they synthesized from a mixture of ZrO.sub.2, WO.sub.3, and NH.sub.4 H.sub.2 PO.sub.4. Evans et al. (1995, Jour. Solid State Chem., 120, 101-104) have shown that the structure of Zr.sub.2 P.sub.2 WO.sub.12 (also referred to as Zr.sub.2 (WO.sub.4)(PO.sub.4).sub.2) is comprised of ZrO.sub.6 octahedra sharing corners with WO.sub.4 and PO.sub.4 tetrahedra. Those workers report that dilatometric and variable temperature X-ray diffractometry studies indicate that Zr.sub.2 P.sub.2 WO.sub.12 exhibits a negative thermal expansion over a broad temperature range. More recently, Evans et al. (1997, Journal Solid State Chem., 133, 580-83) have reported that Zr.sub.2 P.sub.2 WO.sub.12 has a mean lattice CTE of about -30.times.10.sup.-7.degree. C..sup.-1.
The Zr.sub.2 P.sub.2 WO.sub.12 bodies synthesized according to the methods reported in the above literature have porosities greater than about 25%, typically greater than 30%. Such high porosity bodies generally are not useful for industrial applications. Thus, it would be useful to provide a composition having a low, preferably a negative thermal expansion, comprised of Zr.sub.2 P.sub.2 WO.sub.12, or analogues thereof in which Hf is fully or partially substituted for Zr, having a porosity less than about 20%, preferably less than about 10%, and more preferably less than about 5%.
Bodies having a highly negative CTE, such as -30 to -100.times.10.sup.-7.degree. C..sup.-1, can find use as substrates for athermalization of fiber Bragg gratings (FBGs). In the latter application, a FBG is mounted in tension on the negative expansion substrate. Applications of FBGs include passive wavelength division multiplexing and filtering in dense WDM systems, as well as distributed fiber sensors for smart systems to monitor bridges, structures, and highways.
For such applications, variation of the center wavelength of fiber Bragg gratings (FBGs) with respect to temperature, due to thermal expansion of the fiber and variation of the refractive index of the glass, must be minimized. For example, at a Bragg wavelength of 1550 nm, thermal variation of .lambda..sub.B is expected to be 0.012 nm/.degree. C., whereas a value less than 0.002 nm/.degree. C. is desired. Variation in .lambda..sub.B with temperature can be reduced to well below 0.002 nm/.degree. C. by mounting the FBG in tension on a substrate having a negative thermal expansion of about -70 to -85.times.10.sup.-7.degree. C..sup.-1 within that range also -70 to -80.times.10.sup.-7.degree. C., -75 to -82.times.10.sup.-7.degree. C..sup.-1. The reduction in tension with increasing temperature associated with the contraction of the substrate partially or entirely offsets the contribution to increased optical path length resulting from the thermal expansion and change in refractive index of the glass.
.beta.-eucryptite based ceramics formed by controlled devitrification of sintered lithium aluminosilicate glass are being studied as FBG substrates and are disclosed in international patent application no. PCT/US/13062, Beall et al., entitled, "Athermal Optical Device." The attainment of CTEs of -70 to -85.times.10.sup.-7.degree. C..sup.-1 in .beta.-eucryptite bodies requires extensive microcracking; an unmicrocracked .beta.-eucryptite exhibits a CTE near -5.times.10.sup.-7.degree. C..sup.-1. This microcracking results from internal stresses associated with the large difference in CTE along the c and a axes of the crystals (approximately -176 and +78.times.10.sup.-7.degree. C..sup.-1, respectively), coupled with the coarse grain size of the crystals.
U.S. Pat. No. 5,694,503, issued to Fleming et al., discloses using the negative coefficient of thermal expansion material ZrW.sub.2 O.sub.8 to form substrates for temperature compensated fiber Bragg gratings. Since the coefficient of thermal expansion of ZrW.sub.2 O.sub.8 may be too negative to provide temperature compensation for Bragg gratings, the Fleming et al. patent suggests mixing ZrW.sub.2 O.sub.8 with a positive coefficient of thermal expansion material such as alumina, silica, zirconia, magnesia, calcia, or yttria in an amount to raise the coefficient of thermal expansion.
The mixtures of ZrW.sub.2 O.sub.8 with positive coefficient of thermal expansion materials suggested in the Fleming et al. patent, however, have several disadvantages. Large relative differences in the thermal expansion coefficients of ZrW.sub.2 O.sub.8 and the positive CTE materials can cause microcracking in the composite material upon heating and cooling of the material. Such microcracking can result in hysteresis in the thermal expansion curve or dimensional change of the body with changes in humidity, characteristics that are undesirable in a fiber Bragg grating substrate. Furthermore, many of the positive CTE components recommended in the Fleming et al. patent react with the ZrW.sub.2 O.sub.8 during sintering to form copious amounts of liquid. Such reactions and liquid formation tend to cause the body to slump during firing. Alternatively, some of the positive CTE components recommended in the Fleming et al. patent react with the ZrW.sub.2 O.sub.8 to form other high CTE crystalline phases so that the ceramic body does not have the desired strongly negative CTE after firing. In addition, ceramics comprised of ZrW.sub.2 O.sub.8 and ZrO.sub.2 undergo a length change having an absolute value greater than 500 parts per million over 700 hours at 85% relative humidity and 85.degree. C., which is undesirably large.
The presence of microcracking in a FBG substrate requires that the fiber/substrate package be hermetically sealed to prevent dimensional drift of the substrate due to opening and closing of the microcracks resulting from variations in humidity. Hermetic sealing adds significantly to the cost of the assembly, and the reliability of the device becomes dependent upon long-term reliability of the hermetic seal.
Thus, it would be desirable to provide an unmicrocracked material having a porosity less than about 25%, preferably less than about 10% and more preferably less than about 5%. Further, there is a need for a body having a CTE of about -70.times.10.sup.-7.degree. C..sup.-1 to -85.times.10.sup.-7.degree. C..sup.-1 to provide temperature compensation for the gratings of current interest which could be used to make FBG substrates because hermetic sealing would not be required for long-term stability. In addition, it would be desirable to provide a material that has a length change having an absolute value less than 500 ppm over 700 hours at 85.degree. C. and 85% relative humidity.